Under gravel filters (UGF) have fallen seriously out of favor over the last 10-20 years..
most people now consider them the absolute worst kind of filter..
because they are terribly dirty..all that gunk collects under the filter plate where you cant reach it..
it just sits there..forever.
and they have no benefits over cannister or power filters ("hang on the back" filters), which provide just as good mechanical and biological filtration as a UGF.
and cannister and power filters are much cleaner, because you can actually CLEAN them! personally I would never, ever, use a UGF..
they have zero benefits over other filters, and huge drawbacks..
The so-called benefit to a UGF is the massive amount of filter "space", surface area, for the bacteria to live on..yeah, thats true..but the thing is, no aquarium NEEDS that much space!
aquariums do fine with the amount of filter media in a cannister or hanging filter..
my planted tank has been running fine for 10 years with a whisper power filter ("hang on the back" filter) which contains a sponge that is 3X6 inches..its all the biofiltration my tank needs...plus the "good bacteria" also lives in the gravel, on the plants, on every surface in the tank...a tank without a UGF already has WAY more biofiltration potential than it will ever need..
that blows the one and only "advantage" of a UGF out of the water..its "advantage" is simply not needed...and the disadvantage (a thick layer of gunk under the UGF plate)
far outweighs any advantage..and, as I said, there IS no real advantage!
some reading on the UGF debate:
(that article was written in 1986! 22 years ago)
and there is endless discussion on this debate all over the internet..
I fall very firmly on the "NEVER use a UGF" side of the debate.
also, no aquarium "needs" an air bubbler..the fish dont need it.
they get more than enough oxygen without it.
and a planted tank should NOT have one..its a drawback to a planted tank.
airstones are useful for decoration only..if you like to look at the bubbles, thats fine..
but they are not remotely necessary.